One technical field of the invention is the field of scanning probe techniques evolving from the invention of the Scanning Tunneling Microscope (STM) by G. Binnig and H. Rohrer in 1982. The STM, which is described for example in the U.S. Pat. No. 4,343,993, triggered the development of a huge variety of instruments. These instruments are designed to investigate and manipulate surfaces and/or atoms or molecules placed on top of them even with atomic precision. An often used feature of scanning a probe instruments is a fine tip, also called probe, with a radius of curvature at its apex of 100 nm or below. The probe is scanned over the surface of a sample by using coarse- and fine-positioning units. The STM and its derivatives are scientific tools which can be used for all kinds of surface analysis and imaging of submicroscopic phenomena as well as surface modification or lithographic techniques on the nm scale.
Besides the STM, scanning probe techniques include the Atomic Force Microscope (AFM), which was invented by G. Binnig in 1986 (U.S. Pat. No. 4,724,318) and subsequently further developed, as for example described in U.S. Pat. No. 5,144,833. Images of magnetic domains have been obtained by Magnetic Force Microscopy, as described by H. J. Mamin et al. in: Appl. Phys. Lett. 55 (1989), pp. 318ff.
A Scanning Capacitance Microscope is known from the patent U.S. Pat. No. 5,065,103, a Scanning Acoustic Microscope from U.S. Pat. No. 4,646,573, and a Scanning Thermal Profiler from U.S. Pat. No. 4,747,698. The scanning probe technique is also used in light microscopes having a resolution not limited by diffraction. In these so-called Scanning Near-field Optical Microscopes, described for example in U.S. Pat. No. 4,604,520, the probe essentially consists of a light waveguide ending in a tiny aperture which either receives or emits light within the proximity of the surface of a sample.
For the purpose of this invention, all these as well as related techniques are referred to as scanning probe microscopy (SPM). SPM crucially depends on an accurate scanning system sample. By a generally acknowledged convention, the directions within a plane parallel to the surface of the sample are denoted by `x` and `y`, whereas the direction perpendicular to this plane is commonly referred to as `z-axis`.
The scanning system as such has a decisive impact upon the resolution of the scanning probe device. For achieving atomic resolution, it must be able to perform controllable displacements with an accuracy of 0.1 nm or less with the requirements for the z-direction usually being even more vigorous than those for the other directions. An ideal scanning system for scanning probe devices should combine a large scanning range in the x, y-plane with an accurate control of the position of the tip relative to the sample in all three dimensions, in particular in the z-direction. As both requirements are difficult to fulfill, known SPM devices apply two-stage positioning systems: coarse-positioners move the sample close to a desired position on the probe until the distance ranges can be covered by fine-positioners. The fine-positioners alone deliver the required accuracy and thus enable imaging and manipulation with atomic resolution.
Many different approaches and techniques have been applied as a coarse-positioner, including a manual approach using levers or differential springs, a piezo-electric walker mechanism (louse), or a stepping motor coupled to the sample or scanning stage. Magnetic coarse-positioners are described for example in the European patent EP-B-0 290 522 and in the US patent U.S. Pat. No. 4,947,042. The most advanced types of coarse-positioner, in particular when controlled by interferometry, achieve a positioning precision of a few tenths of a micrometer and thus barely overlap with the scanning range of the fine-scanner.
Fine-scanning techniques from the very beginning of scanning probe microscopy converged to using piezoelectric elements. Examples for fine scanning stages are for example known from the patent U.S. Pat. No. 4,520,570 and from G. Binnig and D. P. E. Smith, "Single-tube three-dimensional scanner for scanning tunneling microscopy", published in Rev. Sci. Instruments 57 (1986), p: 1688.
C. Gerber and O. Marti further proposed a magnetostrictive scanner in IBM's Technical Disclosure Bulletin Vol. 27, No. 11, April 1985, p. 6373, in which the piezoelectric elements are replaced by rods made of magnetostrictive material. This material elongates and contracts under the influence of an magnetic field similar to the behavior of a piezoelectric element in an electric field.
It is important for the scope of this invention to notice that even those techniques which apply a magnetism-based coarse-positioning stage, as e.g. described in patents EP-B-0 290 522 and U.S. Pat. No. 4,947,042, rely upon a piezoelectric fine-scanner. Though the piezoelectric fine-scanner is a versatile tool, it exhibits several disadvantages. Primarily, the elongations which can be achieved, range around 2 to 5 nm/V, i.e. a voltage of 1000 V is required to achieve a scan range of 2 to 5 micrometer, which for all practical purpose; limits the range to a few micrometers. In addition, with an increasing voltage, non-linear effects become noticeable with the elongation of the piezoelectric material being no longer proportional to the applied voltage. On the other hand, it requires a complex and accurate control system for applying the necessary voltages to the electrodes attached to the piezoelectric material.
The described examples are meant to illustrate the broad usage of scanning probe techniques but do not cover all applications feasible for the invention. It is, for example, known to a skilled person that the storage density of common storage devices, such as hard disks, is directly dependent on the accuracy at which a write/read head can be positioned relative to the storage medium. An inexpensive, accurate method of positioning the write/read head with atomic precision has an immediate impact in this technical field. In particular, scanning probe storage systems require an accurate scanning system.
A scanning tunneling storage system has been proposed in the European patent EP 247 219, for example. This system comprises current detectors being attached to an array of cantilevers. A storage medium is placed opposite to the array. The storage medium is displaced by means of a two-dimensional piezoelectric positioning device. There is no adequate approach disclosed for erasing the stored information.
U.S. Pat. No. 5,307,311 a memory device is described which makes use of a very large set of independently operating subdevices. It employs an array of hundreds of microcantilevers having an area in which bits are stored. Opposite to these cantilevers there are hundreds of read/write heads which are similar in nature to scanning tunneling or atomic force microscope scanning tips. Each cantilever is moved in an oscillatory manner such that the respective read/write head scans over the bits stored thereon.
A fine-positioning apparatus with atomic resolution is disclosed in the pending PCT patent application PCT/EP 94/02844 entitled "Fine positioning apparatus with atomic resolution". It is a characteristic feature of the fine-positioning apparatus described therein that it comprises an actuator based on the principle of magnetic induction or magnetomotive force. The effect of magnetic induction is characterized by the force that a magnetic field or the change of a magnetic field exerts upon a permanent # magnet, a current-carrying conductor, or an otherwise magnetized material within this field.